1. Technical Field
The present invention relates to positive temperature coefficient (PTC) device and, more particularly, to improved ceramic-based PTC devices and methods of making same.
2. Description of Related Art
As is known in the art, PTC materials exhibit electrical resistivity that increases with increasing temperature. For some PTC materials, electrical resistivity increases sharply above a certain temperature to significantly restrict an electrical current flow through the material. As the PTC material is heated due to electrical current, negative feedback results from increased resistance, which in turn results from the increased material temperature. This feature makes PTC materials suitable for use, for example, in current surge protection devices that limit the electrical current levels that pass through them. Such devices are used to protect electrically powered devices from transient current surges on power supply lines, to protect electrical power sources from overload current drains, or to generally protect electrical equipment in the event that electrical currents exceed design limitations for one reason or another.
Because the temperature rise in a PTC material that results from an electrical current increase is not instantaneous, owing to the thermal mass of the PTC material, the PTC material can also be used to make a time-delayed switch. In a case where the heat produced by an electric current in a PTC material makes it useful as an electrical heating element, the PTC behavior can provide thermostatic self-regulation. Also, PTC devices may be used to sense temperature by measuring the voltage drop across them in response to an electrical current that is low enough to produce negligible self-heating. Some common types of PTC-based electronic components are resettable fuses and thermistors.
Two well known classes of PTC materials are polymer-based and ceramic-based PTC materials.
Many different types of polymers, copolymers, and mixtures of polymers are known in the art as suitable for use in the manufacture of PTC materials. For example, a material of low resistivity particles such as carbon, embedded in a high resistivity organic polymer matrix such as polyethylene, exhibits low electrical resistance at room temperature (e.g., 25° C.) if the concentration of the low resistivity particles is sufficient to form conductive paths through the material. Because the thermal expansion coefficient of the polymer is much greater than that of the low resistivity particles, the polymer matrix expands more than the conductive carbon particles embedded therein when the composite material is heated. Consequently, conductive contact among adjacent carbon particles is diminished as the carbon particles are carried away from one another by the expanding polymer matrix, thereby increasing the electrical resistivity of the composite material.
When an organic substance such as a polymer is used as a high resistivity matrix in a PTC composite material, however, prolonged high temperatures or repeated temperature cycling can degrade the structural integrity of the composite material. This can result in a change of overall resistivity versus temperature characteristics. This may even result in catastrophic failure resulting from excessive heating due to runaway current densities that may be caused by micro-structural failure of the composite material resulting from localized high conductivity, high current regions. This breakdown of polymer-based composite materials is largely due to diminished chemical stability of the polymer material at elevated temperatures. Consequently, conventional polymer composite materials do not allow for reliable repeated operation, because the resistivity characteristic of the material, especially after a trip condition, does not return to its prior state.
Ceramic-based PTC materials, such as barium titanate type ceramics, exhibit sharply increasing resistivity in response to increasing temperature (i.e., PTC behavior) above a certain temperature threshold, and are more chemically and physically stable than polymer-based materials at elevated temperatures. Although ceramic-based PTC materials are more reliable than polymer-based PTC materials, one drawback of ceramic-based PTC materials is that they are characterized by relatively high resistivity (e.g., 30 Ω-cm) at room temperature when compared to polymer-based PTC materials (e.g., 3 Ω-cm). Thus at room temperature operating conditions, for example, ceramic-based PTC materials exhibit a higher power loss than polymer-based PCT materials when conducting the same level of electrical current through devices having the same or similar dimensions. This is a drawback for ceramic-based PTC material devices in many applications where power loss is to be minimized.
One type of composite material that has been proposed to overcome the deficiencies of polymer-based PTC materials and ceramic-based PTC materials such as those discussed above is disclosed in U.S. Pat. No. 6,300,862 to Ishida (hereafter “Ishida”). Ishida describes a PTC composite material that includes a matrix of ceramic material having one of a cristobalite crystal structure and a tridymite crystal structure, each doped with an oxide of at least one of Be, B, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge, and a conductive phase dispersed throughout the matrix. The conductive phase includes at least one of a metal, silicide, nitride, carbide, and boride.
The ceramic material disclosed by Ishida is a special type of ceramic material having a cristobalite or tridymite crystal structure and which is doped with an oxide. This manufactured ceramic material behaves very much like a polymer-based PTC material because when it is heated, the ceramic matrix expands in volume and breaks conductive paths formed by conductive particles dispersed throughout the matrix. In contrast, other types of ceramic PTC materials (e.g., doped barium titanate) do not expand significantly when heated. Although Ishida's composite material exhibits lower room temperature resistance when compared to other ceramic-based PTC materials, it still suffers from many disadvantages as described in the Ishida patent specification. For example, if the volume expansion of the crystal structure ceramic is less than a certain amount, the composite material does not exhibit sufficient resistivity jump at the trip-point temperature. Alternatively, if the volume expansion is more than an upper limit, the composite material may experience stress cracking at the interface between the matrix and the conductive phase. Thus, the manufacture of the ceramic material itself, as well as the manufacture of the overall composite material, requires great care, precision, and expense to ensure that particle sizes are within requisite ranges and the ceramic material exhibits desired expansion characteristics. In sum, the materials and manufacturing process utilized by Ishida are expensive, time consuming, and difficult to consistently repeat for mass production.
U.S. Pat. No. 6,359,327 to Niimi et al. (hereafter “Niimi”) discloses a monolithic PTC device that includes a sintered laminate formed of alternating, stacked semiconductor ceramic layers and interleaved electrode layers. The ceramic layers comprise a sintered barium titanate containing a boron oxide. The internal electrodes are formed from a base metal such as nickel (Ni), copper (Co), iron (Fe) or molybdenum (Mo). A preferred base metal is identified as Ni.
Niimi discloses that the ceramic PTC material comprises various mixtures of BaCO3, Sm2O3, BN and MnCO3 added to the barium titanate to improve its PTC properties. This ceramic material is then used as the ceramic layer of the sintered laminate described above. Niimi further discloses that a monolithic PTC device, having the laminate of alternating stacked ceramic and Ni layers, and external electrodes formed on the laminate, can be efficiently manufactured by co-firing the monolithic device at 950° C. in a hydrogen/nitrogen reducing atmosphere chamber, followed by a second firing at 800° C. in air.
Although the process disclosed by Niimi allows co-firing of an entire monolithic device at relatively low temperature (e.g., 950° C.), this advantage is diminished by the fact that the process requires a reducing atmosphere chamber and related equipment. Such equipment is expensive and difficult to control in terms of maintaining process parameters during operation. Additionally, the process disclosed by Niimi requires a second firing step, which adds to the time and cost of the manufacturing process. Furthermore, the ceramic PTC used by Niimi still suffers from high resistivity (approximately 30 ohm-cm) at room temperature. Therefore, many parallel layers of ceramic PTC material are required to make a ceramic PTC device having a low resistance (e.g., 0.01 to 0.1 Ω-cm) and, consequently, low power consumption.
Although U.S. Pat. No. 6,245,439 to Yamada et al. (hereafter “Yamada”) discloses a thermistor made from a composite material comprising a ceramic material and a metal material, Yamada is concerned primarily with providing composite materials with improved interphase mechanical bonding. Yamada does not address improving the specific electrical/PTC properties of prior PTC materials. Nor does Yamada address the problems associated with prior polymer-based and ceramic-based PTC materials, as discussed above. Nor does Yamada address how to establish strong ohmic bonding between the metal phase and the ceramic PTC phase. Failure to establish such ohmic bonding (or electrical connection) between the metal phase and ceramic phase, results in a high overall resistance of the composite material.
Thus, what is desired is an improved ceramic or ceramic composite PTC device having improved PTC properties. The improved PTC device should exhibit low resistance at room temperature and a large resistance jump at a tripping temperature of the PTC material. Additionally, the improved PTC device should not substantially degrade as a result of prolonged or repeated exposure to a tripping temperature/fault current. It is further desirable that the improved PTC device can be fired at relatively low temperatures (e.g., between 600 and 900° C.) such that the firing can be performed after assembly of a monolithic multi-layer device containing the PTC material and other electrodes that require low co-firing temperatures. It is further desirable that an improved composite PTC material utilizes relatively inexpensive materials and can be fired in relatively low cost furnaces operating in atmospheric conditions.